The Omega class of glutathione transferases (GSTs) have been identified in organisms including human, mouse, rat, pig, Caenorhabditis elegans, and Drosophila melanogaster. These GSTs, which include glutathione S-transferase omega 2 (GSTO2) have poor activity with common GST substrates, but exhibit novel glutathione-dependent thioltransferase, dehydroascorbate reductase, and monomethylarsonate reductase activities, and also modulate calcium release by ryanodine receptors. The GSTO2 gene contains seven exons, the first of which does not contain GSTO2 coding sequence. The GSTO2 gene is separated from the GSTO1 gene by 7.5 kb on chromosome 10q24.3. GSTO2 is expressed at high levels in the testes, and also is expressed in liver, kidney, skeletal muscle, heart, and lung, with low level expression in the cervix, ovary, and prostate. Substrates for GSTO2 include monomethylarsonic acid (MMV), which is reduced from the +V form to the +III form, as well as dehydroascorbate (DHA) and glutathione (GSH), which can be reacted to give ascorbate and glutathione disulfate.